Numerous systems exist where a signal of some type is superimposed on a power signal. In these systems, the communication information is separated from the power signal in the frequency spectrum such that through proper frequency separation techniques and with proper encoding and decoding techniques the information can be extracted from the power signal. These systems can perform very well and meet many design needs. Systems comprising a high frequency carrier, upon which is impressed the encoded information, requires a tightly specified and controlled transmission medium such that the high frequency carrier is not lost and signal integrity is maintained. These systems generally tend to require more complex encoding and decoding hardware. If the proper techniques are not used, transmission distances and data transmission integrity can be limited. If good techniques are used, very high data rates can be achieved.
In the design center, there are often requirements to use existing wiring such as in sprinkler systems where it is both cost effective and much less disruptive to the environment to use existing wiring. A typical system may have an individual wire to each valve and a common ground. This type of lay out is not well suited to high frequency encoding systems where transmission line characteristics are important. With conventional systems, adding additional valves means installing a new wire for each new valve.
Fowler (U.S. Pat. No. 4,093,946) teaches the fundamentals of communication over a two-wire system and method for interrogating a plurality of data gathering devices for actuation of selected ones from which data is received. Transducers, as taught by Fowler, are typically remotely located from the interrogating and receiving apparatus and are connected via a single two-conductor path over which power is conveyed to the transducers and data signals conveyed between the transducers and the receiving apparatus. Fowler teaches that turning on current source and causing a constant current to be propagated down cable can produce a binary signal. He continues that voltage modulation can be can be induced by alternately placing a high impedance and a short circuit across the conductors of the cable, while the current source is continuously enabled. The voltage on the cable can be caused to alternate between two binary states.”
Shimada (U.S. Pat. No. 4,139,737) applies the two-wire communication concept through the addition of a time division multiplex transmission system in which electrical power is transmitted to remote terminals from a central unit simultaneously with address and control signals.
Shimada also references that adding modulators and demodulators to power lines was well known in 1979. “Alternatively a pair of transmission lines have been arranged between the central unit and the respective terminal units and, further, modulators and demodulators have been inserted between the central unit and the said transmission lines and between the respective terminal units and the transmission lines so that the address, control, response and the like signals have been superposed on electric power waves to be transmitted between the central unit and the respective terminal units. However, with this arrangement, the modulators and demodulators have been required, the system formation has been complicated and the cost reduction has not been able to be attained.” This technique is widely used in both AC and DC two-wire systems. Systems with high frequency carriers require special care in impedance matching and layout and are limited in distance. They also require a certain level of cost and complexity.
Horn (U.S. Pat. No. 4,208,650) in 1980 teaches the concept of using a basic message protocol where “The transmitter is connected to a data containing unit and operates through a data cycle which addresses the data unit to provide a plurality of serially arranged message frames during each data cycle. Each message frame includes a marker bit, a sync word, data words having either digital or analog information, address and error words, a checkword, and various parity and start bits. The receiver, which is connected to another data unit, is adapted to recognize each message frame by means of the marker bit and the sync word, and evaluate the message validity by means of the parity and start bits, and the checkword.”
These four patents teach the following principles:                1. Communicating and supplying power over a two-wire system,        2. Impressing a current source on a line and causing modulation on the line by sinking that current such that the voltage will fall to a low level where alternating changes between the high and low levels is detected and decoded by the intended receiver,        3. Each message frame includes a marker bit, a sync word, data words having either digital or analog information, address and error words, a checkword, and various parity and start bits.        
There have been significant refinements and variations over time including the following. Each variation was developed to meet specific requirements demanded by the projected usage.
Demeyer, et al. (U.S. Pat. No. 5,089,974) in 1992 teaches a system similar to the present invention. The Demeyer abstract reads:
“A building power management controller comprises a plurality of modules connected by a two-wire network. Each module comprises a data transceiver device, controlled by a microprocessor, to both transmit data to the other modules and to a central unit via the two-wire network, and to receive information via this two-wire network. The modules are supplied with power by the two-wire network. When two modules transmit simultaneously, one takes priority so as not to disturb the messages transmitted.”
Demeyer utilizes the concept taught by Fowler above, where a current source is applied and an alternate sink and high impedance load is used to generate the timing for the coding scheme. Demeyer also teaches that super imposed communication signals can be handled by sensing an out of phase bit through a comparator means where a difference in state between the output of the transmit bit and the input from the two-wire level sense comparator is sensed. The comparator is will indicate by an output level if there is a difference between the lines while the module is in the low state. The microcontroller in the module can sense this level and withdraw from the communication cycle letting the other device continue.
Demeyer does not teach the ability to use this capability to automatically configure the system.